1. Field of the Invention
The present invention relates generally to motor control circuits and more specifically to motor control circuits configured to control rotational direction of the motor, dynamic braking of the motor, and protection of the motor and motor control circuits from possibly harmful electrical energy.
2. State of the Art
Motor relay arrangements in an H-bridge configuration are conventionally used to control Direct Current (DC) motor direction. In its basic form, an H-bridge circuit typically includes four relays. On one side of the motor, a first relay connects a first motor terminal to a power source or an open circuit and a second relay connects the first motor terminal to ground or an open circuit. On the other side of the motor, a third relay connects a second motor terminal to a power source or an open circuit and a fourth relay connects the second motor terminal to ground or an open circuit. The H-bridge operates to cause current to flow through the motor, and cause forward rotation by energizing the first relay and the fourth relay, which causes current to flow through the first relay, through the motor from the first motor terminal to the second motor terminal, then to ground through the fourth relay. Similarly, to cause a backward rotation, the second relay and the third relay are energized, causing current to flow through the third relay, through the motor from the second motor terminal to the first motor terminal, then to ground through the second relay. Unfortunately, if the wrong combination of relays is energized, too much current may flow through the relays resulting in various problems including, for example, damage to the circuit, the motor, or both.
Various improvements have addressed this problem. One such improvement is U.S. Pat. No. 6,153,990, entitled “Relay Circuit for High Voltage Operation of a Bi-Directional DC Motor.” In this patent, three relays are utilized. A first relay is energized to provide a path to ground anytime a forward signal or a reverse signal is asserted. The path to ground, which is an output of the first relay, connects to a second relay and a third relay. The second relay and third relay are configured to steer the current from a power signal through the motor and to the path to ground. The second relay and third relay steer the current in one direction if a forward signal is asserted and the other direction if a reverse signal is asserted. The first relay additionally prevents excessive current from flowing through the relays by interrupting current to the second relay and third relay any time both the forward signal and the reverse signal are negated.
Unfortunately, because the first relay is energized when the forward signal or the reverse signal is asserted, the current interruption may be circumvented when both the forward signal and the reverse signal are asserted simultaneously. Furthermore, prior art circuits may not protect the H-bridge relays from arcing when the motor is placed in a dynamic braking mode where both terminals of the motor are placed at the same voltage potential.
A need exists to control a DC motor in both the forward rotational direction and the reverse rotational direction, enable dynamic braking of the motor, improve the performance of dynamic braking, and protect the contacts of the relays in the motor control circuit. In addition, a need exists for the motor control circuit to operate appropriately while also preventing damage to the components of the motor or the control circuitry for all logical combinations of control signals for forward and reverse rotational directions.